This proposal addresses the molecular mechanism whereby the Ewing sarcoma fusion protein EWS/FLH leads to chromosome instability and malignant transformation. Ewing sarcoma is the second most common form of bone cancer in children. EWS/FLH is a chimeric fusion protein containing EWS-derived sequences at the amino-terminal region fused to the carboxy-terminal regions of the ETS transcription factor FLU. FLU is essential for hemangioblast differentiation, whereas the in vivo function of EWS is not well understood. In addition to EWS/FLH in Ewing sarcoma, EWS was shown to fused to a number of different genes in other sarcomas. There are a number of unanswered questions regarding the role of EWS/FLH in Ewing sarcoma, including: Which cellular function(s) of EWS/FLH cause malignant transformation? Is expression of EWS/FLH alone sufficient for Ewing sarcoma formation? And why do tumors develop mainly in skeletal elements? To date, there is no animal model for Ewing sarcoma because either the toxicity of the EWS/FLH fusion protein leads to embryonic lethality, or because of failure to exhibit the Ewing sarcoma phenotype. We previously reported that both the knockdown of endogenous EWS, and expression of the EWS/FLH fusion protein in zebrafish embryos and HeLa cells lead to mitotic defects. And the interaction between EWS/FLH and wildtype EWS leads to inhibition of EWS activity. The hypothesis of this proposal is that EWS/FLH interaction with endogenous EWS induces mitotic defects leading to chromosome instability and to malignant transformation. This proposal is designed to address the following aims: Aim 1: To determine how expression of EWS/FLH and knockdown of EWS lead to mitotic defects. Aim 2: To determine whether knockdown of EWS leads to chromosome instability and Ewing sarcoma in zebrafish. Aim 3: To determine whether expression of EWS/FLH leads to chromosome instability and Ewing sarcoma in zebrafish. We have an EWS mutant zebrafish line and have generated transgenic zebrafish lines expressing EWS/FLH conditionally. This approach will allow us to analyze the tissue-specific effects of EWS/FLH expression and EWS knockdown. This knowledge is essential to understand why Ewing sarcoma develops in a tissue-specific (the skeletal elements) manner. We will conduct a detailed, cell-specific analysis of the effects of expression of EWS/FLH or knockdown of EWS, at multiple stages during development. We will continue to develop and analyze a zebrafish model for Ewing sarcoma that fulfills the long-term goals of 1) understanding the transformation process, 2) identifying any secondary mutations that may be required for Ewing sarcoma formation, and 3) screening for chemical compounds that suppress cancer formation. This study may have an impact on public health because it could lead to the discovery of biomarkers of cancer and drugs for patient therapy.